Intramolecular and intermolecular N-H···F-C hydrogen-bonding interactions in amine adducts of tris(pentafluorophenyl)borane and tris(pentafluorophenyl)alane

The reaction between B(C6F5)3 and NH3(g) in light petroleum yielded the solvated adduct H3N · B(C6F5)3· NH3. Treatment with a second equivalent of B(C6F5)3 afforded H3N · B(C6F5)3. Attempts to prepare the analogous alane adduct were unsuccessful and resulted in protolysis. Related compounds of the form R'R"N(H)· M(C6F5)3 were synthesized from M(C6F5)3 and the corresponding primary and secondary amines (M = B, Al; R' = H, Me, CH2Ph; R" = Me, CH2Ph, CH(Me)(Ph); R'R" = cyclo-C5H10). The solid-state structures of 13 new compounds have been elucidated by single-crystal X-ray diffraction and are discussed. Each of the borane adducts has a significant bifurcated intramolecular hydrogen bond between an amino hydrogen and two o-fluorines, while N-H···F-C interactions in the alane adducts are weaker and more variable. F-19 NMR studies demonstrate that the borane adducts retain the bifurcated C-F···H···F-C hydrogen bond in solution. Compounds of the type R'R"N(H)·M(C6F5)3 conform to Etter's rules for the prediction of hydrogen-bonding interactions